I bought a lumbar support at the dollar store on another member's recommendation and for $1, it is absolutely perfect for trips and things when I will be in the car for a while, for daily driving, I just suck it up.

Several mercedes guys and I think a few XF guys have had an upholstery shop install a manual lumbar support, basically an inflatable bladder that sits inside the leather and a tube with a bulb on the end for infalting and deflating, similar to a doctor's blood pressure cuff. A slick way to do it...